bcftbx/simple_xls: fixes for Python2/3 compatibility

[pjbriggs]

PR which implements a number of updates to the bcftbx/simple_xls module for Python 2/3 compatibility:

• Switch column sorting to use sorted(...,key=...)
• Reimplement the column indices comparison function to drop the cmp builtin (which is unavailable in Python3)
• Switch from xrange to Python3-compatible version of range
• Implement the __bool__ method on the XLSStyle class in parallel with the existing __nonzero__ method
• Implement the __next__ method on the ColumnRange iterator class in parallel with the existing next method
• Fix the integer division operator (i.e. use // rather than /)